Dimictic lake

Dimictic lakes mix from the surface to bottom twice each year. Dimictic lakes are holomictic, a category which includes all lakes which mix one or more times per year. During winter, they are covered by ice. During summer, they are thermally stratified, with temperature-derived density differences separating the warm surface waters (the epilimnion), from the colder bottom waters (the hypolimnion). Such lakes are common in mid-latitude regions with temperate climates.[1]

Examples of dimictic lakes

• Lake Mendota
• Lake Superior
• Lake Simcoe
• Lake Opeongo

Seasonal cycles of mixing and stratification

Typical mixing pattern for a dimictic lake

Mixing (overturning) typically occurs during the spring and autumn, when the lake is "isothermal" (i.e. at the same temperature from the top to the bottom). At this time, the water throughout the lake is near 4 °C (the temperature of maximum density), and, in the absence of any temperature or density differences, the lake readily mixes from top to bottom. During winter any additional cooling below 4 °C results in stratification of water column, so dimictic lakes usually have an inverse thermal stratification, with water at 0 °C below ice and then with temperatures increasing to near 4 °C at the lake's base.[2]

Spring overturn

Once the ice melts, the water column can be mixed by the wind. In large lakes the upper water column is often below 4 °C when the ice melts, so that spring is characterized by continued mixing by solar driven convection,[3][4] until the water column reaches 4 °C. In small lakes, the period of spring overturn can be very brief,[5] so that spring overturn is often much shorter than the fall overturn. As the upper water column warms past 4 °C a thermal stratification starts to develop.

Summer stratification

During summer, the heat fluxes from the atmosphere to a lake warms the surface layers. This results in dimictic lakes have a strong thermal stratification, with a warm epilimnion separated from the cold hypolimnion by the metalimnion. Within the metalimnion there is a thermocline, usually defined as the region where temperature gradients exceed 1 °C/m.[6] Due to the stable density gradient, mixing is inhibited within the thermocline,[7] which reduces the vertical transport of dissolved oxygen. If a lake is eutrophic and has a high sediment oxygen demand, the hypolimnion in dimictic lakes can become hypoxic during summer stratification, as often seen in Lake Erie.

During summer stratification, most lakes are observed to experience internal waves due to energy input from winds. If the lake is small (less than 5 km in length), then the period of the internal seiche is well predicted by the Merian formulae.[8] Long period internal waves in larger lakes can be influenced by Coriolis forces (due to the Earth's rotation). This is expected to occur when the period of internal seiche becomes comparable to the local inertial period, which is 16.971 hours at a latitude of 45 °N (link to Coriolis utility). In large lakes (such a Lake Simcoe, Lake Geneva, Lake Michigan or Lake Ontario) the observed frequencies of internal seiches are dominated by Poincaré waves[9][10] and Kelvin waves.[11][12]

Fall overturn

In late summer, air temperatures drop and the surface of lakes cool, resulting in a deeper mixed layer, until at some point the water column becomes isothermal, and generally high in dissolved oxygen. During fall a combination of wind and cooling air temperatures continue to keep the water column mixed. The water continues to cool until the temperature reaches 4 °C. Often fall overturn can last for 3–4 months.

Winter inverse stratification

After the water column reaches the temperature of maximum density at 4 °C, any subsequent cooling produces less dense water due to non-linearity of equation of state of water. Early winter is thus a period of restratification.[13] Once ice forms on a lake, the heat fluxes from the atmosphere are largely shut down. The development of thermal stratification during winter is then defined by two periods: Winter I and Winter II.[14] During the early winter period of Winter I the major heat flux is due to heat stored in sediment; during this period the lake heats up from beneath forming a deep layer of 4 °C water.[14] During late winter, the surface ice starts to melt and with the increased length of the day, there is increased sunlight that penetrates through the ice into the upper water column. Thus during Winter II, the major heat flux is now from above, and the warming causes an unstable layer to form, resulting in solar driven convection.[4][15][2] This mixing of the upper water column is important for keeping plankton in suspension[16][2][17]

See also

• Amictic
• Holomictic
• Meromictic
• Monomictic
• Polymictic
• Thermocline

References

1. Lewis, William M., Jr. (1983). "A revised classification of lakes based on mixing" (PDF). Canadian Journal of Fisheries and Aquatic Sciences. 40 (10): 1779–1787. doi:10.1139/f83-207. Archived from the original (PDF) on 2009-03-06.
2. Yang, Bernard; Young, Joelle; Brown, Laura; Wells, Mathew (2017-12-23). "High-Frequency Observations of Temperature and Dissolved Oxygen Reveal Under-Ice Convection in a Large Lake". Geophysical Research Letters. 44 (24): 12, 218–12, 226. Bibcode:2017GeoRL..4412218Y. doi:10.1002/2017gl075373. ISSN 0094-8276.
3. Cannon, D. J.; Troy, C. D.; Liao, Q.; Bootsma, H. A. (2019-06-28). "Ice‐Free Radiative Convection Drives Spring Mixing in a Large Lake". Geophysical Research Letters. 46 (12): 6811–6820. Bibcode:2019GeoRL..46.6811C. doi:10.1029/2019gl082916. ISSN 0094-8276.
4. Austin, Jay A. (2019-04-22). "Observations of radiatively driven convection in a deep lake". Limnology and Oceanography. 64 (5): 2152–2160. doi:10.1002/lno.11175. ISSN 0024-3590.
5. Pierson, D.C.; Weyhenmeyer, G. A.; Arvola, L.; Benson, B.; Blenckner, T.; Kratz, T.; Livingstone, D.M.; Markensten, H.; Marzec, G.; Pettersson, K.; Weathers, K. (February 2011). "An automated method to monitor lake ice phenology". Limnology and Oceanography: Methods. 9 (2): 74–83. doi:10.4319/lom.2010.9.0074. ISSN 1541-5856.
6. Gorham, Eville; Boyce, Farrell M. (January 1989). "Influence of Lake Surface Area and Depth Upon Thermal Stratification and the Depth of the Summer Thermocline". Journal of Great Lakes Research. 15 (2): 233–245. doi:10.1016/s0380-1330(89)71479-9. ISSN 0380-1330.
7. Chowdhury, Mijanur R.; Wells, Mathew G.; Cossu, Remo (December 2015). "Observations and environmental implications of variability in the vertical turbulent mixing in Lake Simcoe". Journal of Great Lakes Research. 41 (4): 995–1009. doi:10.1016/j.jglr.2015.07.008. ISSN 0380-1330.
8. Mortimer, C. H. (January 1974). "Lake hydrodynamics". SIL Communications, 1953-1996. 20 (1): 124–197. doi:10.1080/05384680.1974.11923886. ISSN 0538-4680.
9. Choi, Jun; Troy, Cary D.; Hsieh, Tsung-Chan; Hawley, Nathan; McCormick, Michael J. (July 2012). "A year of internal Poincaré waves in southern Lake Michigan". Journal of Geophysical Research: Oceans. 117 (C7): n/a. Bibcode:2012JGRC..117.7014C. doi:10.1029/2012jc007984. hdl:2027.42/95363. ISSN 0148-0227.
10. Chowdhury, Mijanur R.; Wells, Mathew G.; Howell, Todd (April 2016). "Movements of the thermocline lead to high variability in benthic mixing in the nearshore of a large lake". Water Resources Research. 52 (4): 3019–3039. Bibcode:2016WRR....52.3019C. doi:10.1002/2015wr017725. ISSN 0043-1397.
11. Flood, Bryan; Wells, Mathew; Dunlop, Erin; Young, Joelle (2019-08-14). "Internal waves pump waters in and out of a deep coastal embayment of a large lake". Limnology and Oceanography. 65 (2): 205–223. doi:10.1002/lno.11292. ISSN 0024-3590.
12. Bouffard, Damien; Lemmin, Ulrich (December 2013). "Kelvin waves in Lake Geneva". Journal of Great Lakes Research. 39 (4): 637–645. doi:10.1016/j.jglr.2013.09.005. ISSN 0380-1330.
13. Farmer, David M.; Carmack, Eddy (November 1981). <1516:wmaria>2.0.co;2 "Wind Mixing and Restratification in a Lake near the Temperature of Maximum Density". Journal of Physical Oceanography. 11 (11): 1516–1533. Bibcode:1981JPO....11.1516F. doi:10.1175/1520-0485(1981)011<1516:wmaria>2.0.co;2. ISSN 0022-3670.
14. Kirillin, Georgiy; Leppäranta, Matti; Terzhevik, Arkady; Granin, Nikolai; Bernhardt, Juliane; Engelhardt, Christof; Efremova, Tatyana; Golosov, Sergey; Palshin, Nikolai; Sherstyankin, Pavel; Zdorovennova, Galina (October 2012). "Physics of seasonally ice-covered lakes: a review". Aquatic Sciences. 74 (4): 659–682. doi:10.1007/s00027-012-0279-y. ISSN 1015-1621. S2CID 6722239.
15. Bouffard, Damien; Wüest, Alfred (2019-01-05). "Convection in Lakes" (PDF). Annual Review of Fluid Mechanics. 51 (1): 189–215. Bibcode:2019AnRFM..51..189B. doi:10.1146/annurev-fluid-010518-040506. ISSN 0066-4189.
16. Kelley, Dan E. (1997). "Convection in ice-covered lakes: effects on algal suspension". Journal of Plankton Research. 19 (12): 1859–1880. doi:10.1093/plankt/19.12.1859. ISSN 0142-7873.
17. Bouffard, Damien; Zdorovennova, Galina; Bogdanov, Sergey; Efremova, Tatyana; Lavanchy, Sébastien; Palshin, Nikolay; Terzhevik, Arkady; Vinnå, Love Råman; Volkov, Sergey; Wüest, Alfred; Zdorovennov, Roman (2019-02-19). "Under-ice convection dynamics in a boreal lake". Inland Waters. 9 (2): 142–161. doi:10.1080/20442041.2018.1533356. ISSN 2044-2041.

External links

• "Circulation: annual patterns of dimictic lakes" at Encyclopædia Britannica Online